It is no longer possible to imagine modern semiconductor technology without semiconductor capacitors. Important examples of use of semiconductor capacitors are dynamic random access memories (DRAM) in which the semiconductor capacitors are used as memory cells, and metal oxide semiconductor field effect transistors (MOSFETs) in which the substrate, the gate electrode and the gate oxide between the substrate and the gate electrode form a semiconductor capacitor.
Like all capacitors, it holds good for a semiconductor capacitor that the capacitance of the capacitor is proportional to the dielectric constant of the dielectric between the capacitor electrodes and the area of the capacitor electrodes as well as the reciprocal value of the spacing between the capacitor electrodes, that is to say the thickness of the dielectric. Silicon oxide (SiO2) is frequently used as the dielectric in the semiconductor art.
With the increasing reduction in the component size in the semiconductor art, the dimensions of the capacitor plates of semiconductor capacitors, for example the gate electrodes of MOSFETs, are also progressively decreasing. This means however that the capacitance of the semiconductor capacitor is also reduced unless measures are taken to counteract that.
There are two possible ways of compensating for the reduction in the dimensions of the capacitor electrodes. The first option involves reducing the thickness of the dielectric. For example in MOSFETs in which silicon oxide is typically used as the dielectric, that gives rise to problems, with gate lengths of less than 0.1 μm. The silicon oxide for components with such short gate lengths would then have to be thinner than 1.5 nm. Such a thin silicon oxide however results in an increase in the leakage current of the MOSFET. The leakage current occurs by virtue of electrons which tunnel through the thin gate oxide between the substrate and the gate electrode. The number of tunnelling electrodes and thus the strength of the leakage current increases exponentially with a progressively decreasing silicon oxide layer thickness. It is desirable however to minimise the leakage current of an MOSFET as the aim is to consume as little electrical power as possible for controlling the current between the drain electrode and the source electrode.
The second possible way of compensating for the reduction in the capacitor electrode area involves altering not the thickness of the dielectric but the dielectric constant thereof. If for example praseodymium oxide (Pr2O3) is used as the dielectric instead of silicon oxide, the capacitance of the capacitor can be markedly increased, with the parameters involved being otherwise the same, by virtue of the praseodymium oxide having a higher dielectric constant than silicon oxide. Silicon oxide has a dielectric constant of 3.9 whereas praseodymium oxide has a dielectric constant of 30. This means that, with praseodymium oxide as the dielectric, the gate oxide can be thicker than a dielectric of silicon oxide by the factor of 30 divided by 3.9. Therefore, with praseodymium oxide as the gate dielectric, the leakage current can be drastically reduced in comparison with silicon oxide as the dielectric.
The thickness of a silicon oxide layer which, with a constant area in respect of the capacitor electrodes, affords the same capacitance as the praseodymium oxide layer, is referred to hereinafter as the equivalent oxide layer thickness. With an increasing reduction in component size, that equivalent oxide layer thickness must be reduced in order to compensate for the reduction in the capacitor electrode area. By means of praseodymium oxide as the dielectric, it is possible to increase the actual oxide layer thickness in comparison with the equivalent oxide layer thickness, and thus reduce the tunnelling leakage current.
Praseodymium oxide is typically deposited by vapor deposition of Pr6O11 on silicon. In that case, a mixed oxide of the form (PrO2)x(SiO2)1−x with O<×<1, typically in non-stoichiometric form, is formed between the silicon and the praseodymium oxide (Pr2O3). Thermal process steps following the deposition procedure additionally result in further spreading of the mixed oxide, in particular the SiO2 component.
The mixed oxide has a lower dielectric constant than the pure praseodymium oxide, whereby the equivalent oxide layer thickness of the dielectric is increased in comparison with a pure praseodymium oxide dielectric. The mixed oxide therefore worsens the electrical properties of the dielectric and thus the semiconductor capacitor, more specifically to a greater degree in proportion to an increasing thickness of the mixed oxide.